MR-PET imaging system integration

ABSTRACT

A data processing unit for an integrated magnetic resonance (MR) and positron emission tomography (PET) system includes an RF shield housing, a first input port in the RF shield housing configured to receive a PET detector signal, a first filter disposed in the RF shield housing, in communication with the first input port, and configured to remove MR noise from the PET detector signal, a second input port in the RF shield housing configured to receive DC power, a second filter disposed in the RF shield housing, in communication with the second input port, and configured to remove the MR noise from the DC power, and a signal processing circuit disposed in the RF shield housing and powered by the DC power, the signal processing circuit including an analog-to-digital converter to digitize the PET detector signal.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application claims the benefit of U.S. provisional applicationsentitled “Power Distribution for Hybrid Imaging,” filed Jul. 21, 2010,and assigned Ser. No. 61/366,267, and entitled “Board-Level Partitioningfor Hybrid Imaging,” filed Jul. 21, 2010, and assigned Ser. No.61/366,272, the entire disclosures of which are hereby incorporated byreference.

BACKGROUND

The present embodiments relate to integration of imaging systems,specifically magnetic resonance (MR) imaging and positron emissiontomography (PET) systems.

PET may be combined with another imaging modality in a multimodalitysystem. Such multimodality imaging systems may have diagnostic andbusiness value. Both PET/computed tomography (CT) and single photonemission computed tomography (SPECT)/CT multimodality imaging systemsallow scans to be performed back-to-back or in a same coordinate systemand with similar timing. The axial fields of view of the individualmodalities are as close together as possible in order to minimize theimpact of patient motion and increase spatial correlation of therespective data sets.

Another hybrid example is a brain scan PET system integrated with amagnetic resonance (MR) system. In order for the MR and PET fields ofview to overlap, the PET detectors are placed as an insert in front ofthe body coil. The MR body coil is used to excite the molecules of thepatient by delivering an RF burst. The MR switches into a receive mode,after delivery of the RF burst, and detects RF signals emitted from thepatient. The signal-to-noise ratio of the MR received signal is animportant aspect of MR imaging. The signal-to-noise ratio is importantenough that a typical MR system is enclosed in a radio frequency (RF)cabin that suppresses RF signals, such as by 100 dB, for both externalsignals entering the RF cabin and internal signals exiting the RF cabin.

Electromagnetic interference (EMI) and electromagnetic compatibility(EMC) between the MR and PET subsystems is one of the dominant technicalchallenges facing MR/PET integration. The MR subsystem is extremelysensitive to any RF emissions from the PET subsystem, near the hydrogenspin frequency (e.g., roughly 123 MHz+/−500 KHz for a 3 Tesla system).Likewise, the PET front end is extremely vulnerable to the RF emissionsfrom the MR subsystem. Coincidence windows of 4-10 nS are typical ofnon-time-of-flight PET scanners, which corresponds with a PET signalchain stable to 100 pS. For the brain scan PET/MR system, the detectorsignals are routed out of the RF cabin to avoid EMI and EMC issues withthe MR subsystem. Outside the cabin, the signals are amplified andfiltered. However, the length of the cabling may have detrimentaleffects on signal integrity and timing precision for the PET subsystem.The volume and weight of the cabling may introduce other complications,including performance limitations through restrictions on the number ofPET detectors in the integrated system.

SUMMARY

By way of introduction, the embodiments described below include systems,devices, and methods for supporting the integration of components of amagnetic resonance (MR) subsystem and a positron emission tomography(PET) subsystem. A data processing unit configured to process PETdetector signals includes various components to support suchintegration. The data processing for PET detection may be provided by aplurality of modular data processing units, such as units in discreteradio frequency (RF) housings positioned within a RF cabin of the MRsubsystem.

In a first aspect, a data processing unit for an integrated magneticresonance (MR) and positron emission tomography (PET) system includes anRF shield housing, a first input port in the RF shield housingconfigured to receive a PET detector signal, and a first filter disposedin the RF shield housing. The first filter is in communication with thefirst input port, and is configured to remove MR noise from the PETdetector signal. The data processing unit further includes a secondinput port in the RF shield housing configured to receive DC power and asecond filter disposed in the RF shield housing. The second filter is incommunication with the second input port, and is configured to removethe MR noise from the DC power. The data processing unit furtherincludes a signal processing circuit disposed in the RF shield housingand powered by the DC power, the signal processing circuit including ananalog-to-digital converter to digitize the PET detector signal.

In a second aspect, an integrated magnetic resonance (MR) and positronemission tomography (PET) system includes an MR scanner including amagnet that defines an opening in which a subject is positioned, the MRscanner shielded from external electromagnetic interference, a set ofPET detectors disposed within the opening, each PET detector beingconfigured to generate a PET detector signal, and a plurality of dataprocessing units, each data processing unit being configured forcommunication with a respective one or more PET detectors of the set ofPET detectors. Each data processing unit includes digitization circuitryconfigured to quantify the PET detector signals from the one or more PETdetectors in communication with the data processing unit. Each dataprocessing unit further includes a DC power input port. Each dataprocessing unit further includes a filter in communication with the DCpower input port and configured to block a frequency band used by the MRscanner during the scan operation.

In a third aspect, a method of positron emission tomography (PET)imaging includes receiving PET detector signals via a first input portin an RF shield housing, receiving a DC power supply through a secondinput port in the RF shield housing, filtering the DC power supply toremove noise, and digitizing the PET detector signals usinganalog-to-digital converters disposed in the RF shield housing using thefiltered DC power supply.

The present invention is defined by the following claims, and nothing inthis section should be taken as a limitation on those claims. Furtheraspects and advantages of the invention are discussed below inconjunction with the preferred embodiments and may be later claimedindependently or in combination.

BRIEF DESCRIPTION OF THE DRAWINGS

The components and the figures are not necessarily to scale, emphasisinstead being placed upon illustrating the principles of the invention.Moreover, in the figures, like reference numerals designatecorresponding parts throughout the different views.

FIG. 1 is a block diagram of an example embodiment of an integratedMR/PET imaging system.

FIG. 2 is a block diagram of data acquisition components of a PETsubsystem within an RF cabin according to one embodiment.

FIG. 3 is a perspective view of an integrated MR/PET scanner accordingto one embodiment.

FIG. 4 is a perspective view of a PET subsystem of the integrated MR/PETscanner of FIG. 3 according to one embodiment.

FIG. 5 is a block diagram of a data acquisition unit of the PETsubsystem of FIG. 4 according to one embodiment.

FIG. 6 is a side, elevation view of the PET data acquisition unit of thePET subsystem of FIG. 4 according to one embodiment.

FIG. 7 is a perspective view of an integrated MR/PET scanner accordingto another embodiment.

FIG. 8 is a perspective view of a filter panel at an RF cabin interfaceaccording to one embodiment of the disclosed integrated MR/PET imagingsystems.

FIG. 9 is a perspective view of a modular power supply andcommunications rack according to one embodiment of the disclosedintegrated MR/PET imaging systems.

FIGS. 10A and 10B are schematic top and bottom views of a printedcircuit board (PCB) assembly of a PET data acquisition unit according toone embodiment.

FIG. 11 is a schematic, plan view of a ground partition of a printedcircuit board (PCB) assembly according to one embodiment.

FIG. 12 is a partial, cross-sectional, schematic view of a chassis orhousing of a PET data acquisition unit according to one embodiment.

FIG. 13 is an exploded, schematic view of a plurality of layers of aprinted circuit board (PCB) assembly according to one embodiment.

DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS

An integrated PET/MR system is provided for whole body or other imaging.PET and MR image data may be acquired simultaneously with overlapping orthe same volumetric fields of view. Signals from PET detectors, such asavalanche photodiodes, are filtered, amplified, digitized, and otherwiseprocessed inside the RF cabin. Addressing these data processingfunctions inside the RF cabin may allow the length of the interconnectsbetween PET detectors and PET digitization electronics to be minimized.Such processing of the PET detector signals may maintain PET signal dataintegrity, improve PET data timing precision, and/or otherwise reducesignal losses or other distortion. Such processing may reduce cablingrequirements, relative to integrated systems that process the PETdetector signals outside the RF cabin.

An integrated MR/PET imaging system includes a number of PET subsystemcomponents disposed within an MR environment (e.g., an RF cabin). Dataprocessing and other aspects of the PET subsystem occur within the RFcabin despite the strong magnetic fields presented by the MR environmentand despite the intolerance of the MR subsystem for RF noise. The PETsubsystem includes a number of data processing units configured with anRF shield housing for compatibility with the main magnet, gradient, andRF excitation fields of the MR subsystem. The RF shield housing of eachdata processing unit is also configured to block the RF emissions of theenclosed electronics and avoid interfering with the MR imaging. Locatingthese PET subsystem components inside the RF cabin may allow the dataprocessing and other PET operations to be implemented in close proximityto the PET detectors. The integrated MR/PET imaging system may include athermal management apparatus to manage the heat generated by the PETsubsystem components within the RF cabin.

The PET subsystem may be configured in a modular architecture. Each dataprocessing unit may be discrete from the other data processing units.Each data processing unit may be galvanically isolated from the otherdata processing units via a modular power distribution system. Two ormore data processing units are provided, such as tens or hundreds. Inalternative embodiments, a single data processing unit and housing areprovided.

The data processing units of the PET subsystem may be disposed in avariety of arrangements within the RF cabin. Deployment options includenear the MR scanner or main magnet (e.g., near the magnet bore) in asymmetrical star, near the MR scanner or magnet in one or more clusters,alongside the MR scanner or magnet in symmetrical columns, or spacedfrom the MR scanner within the RF cabin. Although described inconnection with a number of examples in which the data processing unitsare located adjacent the MR scanner or main magnet, the data processingunits may be disposed in a variety of locations within the RF cabin,including, for example, some locations closer to an RF filter plate thatprovides an interface with the RF cabin. The disclosed systems mayinclude a gantry or other apparatus for adjusting the location of thedata processing units and/or other components of the PET subsystemwithin the RF cabin. The spacing between the data processing units andthe MR scanner, main magnet, or magnet opening (e.g., bore) may be fixedor adjusted.

The disclosed systems and methods are not limited to use with anyparticular MR subsystem or primary magnet type. The MR subsystem neednot include a cryomagnet or superconducting or other electromagnet. TheMR subsystem may have a tunnel or open configuration. Othercharacteristics of the MR subsystem may also vary, including theconstruction and components of the RF cabin.

FIG. 1 shows a hybrid magnetic resonance (MR) and positron emissiontomography (PET) system 10. The hybrid PET/MR system 10 includes PET andMR portions or subsystems. Only parts of the PET subsystem and parts ofthe MR subsystem are shown. Additional, different, or fewer componentsmay be provided. The MR and PET subsystems include scanners or scanningcomponents located within an RF cabin 12 that defines an imaging orscanning environment for each subsystem. The RF cabin 12 may be a roomor other enclosure configured with one or more RF shields to reduce RFnoise within the environment. The RF cabin 12 may be a volume of anydesired shape or size isolated by a Faraday cage. The construction andother characteristics of the RF cabin 12 and the RF shields of the RFcabin 12 may vary considerably.

The PET and MR subsystems may include a number of components locatedoutside of the RF cabin 12. The system 10 includes MR subsystemequipment 14 and PET subsystem equipment 16, which may be located inrespective equipment cabinets or other enclosures. In this example,separate cabinets for the equipment 14, 16 are located in an equipmentroom 18 outside of the RF cabin 12. The system 10 includes consoles orother user interfaces 20, 22 for the MR and PET subsystems,respectively, which may be located in an operator or user control roomoutside of the RF cabin 12. Each console 20, 22 is in communication withthe subsystem equipment 14, 16 to control the operation of the system10. Different divisions of the parts of the MR and/or PET subsystemswithin and outside of the RF cabin 12 may be provided.

The subsystem equipment 14, 16 may include any number of respectiveprocessors 26, 28, configured to control and communicate with thescanning components of the system 10 inside the RF cabin 12. Theprocessors 26, 28 may include, for instance, a coincidence processor forthe PET subsystem. Other processors may be configured to provideacquisition control for the MR and PET subsystems or PET imagereconstruction. The subsystem equipment 14, 16 may include a number ofdevices for developing the imaging data, such as RF receivers for the MRsubsystem. The subsystem equipment 14, 16 may also include one or morecomponents or devices directed to DC and other power distribution tothese devices. In this example, the subsystem equipment receives linepower (e.g., 480 Volts, three phase).

Inside the RF cabin 12, the MR and PET scanners or image capturecomponents may be integrated into a single freestanding scanner unit tobe occupied by a subject to be imaged. A number of components of thescanner unit may be disposed within a common housing. For example, thescanning components of the system 10 shown in FIG. 1 may be packaged ina single common enclosure. The single common enclosure may include aplurality of housings or shields that fit together or adjacent to eachother. In other embodiments, the scanning components of the integratedPET/MR system 10 are disposed in multiple housings or separatefreestanding units.

The scanning components of the PET subsystem include a set of PETdetectors 30, which may be, for instance, whole body detectors.Additional, different, or fewer PET-related components may be provided.Any now known or later developed PET imaging system components may beused with the modifications discussed herein. The PET detectors 30 arecoupled to and in communication with other parts of the PET subsystem ina manner that allows the MR and PET subsystems to be integrated whileminimizing image data distortion or other loss. Such connections withthe PET subsystem are directed to a variety of functions, including, forinstance, power supply, control and other data communication, image dataprocessing, system clock signals, and cooling.

In nuclear medicine imaging, such as PET, radioactive tracer isotopes,or radiopharmaceuticals, are taken internally, for example intravenouslyor orally. As the radioisotope undergoes positron emission decay (alsoknown as positive beta decay), the radioisotope emits a positron, anantiparticle of the electron with opposite charge. The emitted positrontravels in tissue for a short distance, during which time the positronloses kinetic energy, until the positron decelerates to a point wherethe positron interacts with an electron. The encounter annihilates bothelectron and positron, producing a pair of annihilation (gamma) photonsmoving in approximately opposite directions. These events are detectedwhen the gamma radiation reaches a crystal scintillator in the PETdetector 30, creating a burst of light which is detected byphotomultiplier tubes or silicon avalanche photodiodes (Si APD) in thePET detector 30. The PET detectors 30 capture data representing theradiation emitted, directly or indirectly, by the radiopharmaceuticals.The PET subsystems forms images from the captured data.

The MR scanning components in the RF cabin 12 include a main magnet 32,gradient coils 34, a body coil 36, and a patient bed 38. Additional,different, or fewer components may be provided. Other parts of the MRsubsystem may be provided within a same housing shared by these MRcomponents, within a same room (e.g., within the RF cabin 12), or withthe other MR subsystem equipment 14. The other scanning parts of the MRsubsystem may include local coils, cooling systems, pulse generationsystems, image processing systems, and user interface systems. Any nowknown or later developed MR imaging system may be used with themodifications discussed herein. The location of the different componentsof the MR subsystem (e.g., inside or outside the RF cabin) may vary,with the image processing, tomography, power generation, and userinterface components being, for instance, outside the RF cabin 12. Powercables, cooling lines, and communication cables connect the pulsegeneration, magnet control, and detection systems within the RF cabin 12with the components outside the RF cabin 12 through a filter plate.

The configuration of the main magnet 32 may vary. The main magnet may bea cryomagnet (e.g., a superconducting or other electromagnet) or a fixedor permanent magnet. The main magnet 32 and other MR scanning componentsmay be configured to have a tubular bore, a laterally open examinationsubject bore, or any other opening defining a field of view. The patientbed 38 (e.g., a patient gurney or table) supports an examination subjectsuch as, for example, a patient. The patient bed 38 may be moved intothe examination subject bore in order to generate images of the patient.In one embodiment, a local coil arrangement for acquiring signals from alocal region (e.g., the head) may be placed on or adjacent to thepatient. Received signals may be transmitted by the local coilarrangement via, for example, coaxial cable or radio link (e.g., viaantennas) for image generation.

In order to examine the patient using the MR portion, different magneticfields are temporally and spatially coordinated with one another forapplication to the patient. The main magnet 32 generates a strong staticmain magnetic field B0 in the range of, for example, 0.2 Tesla to 3Tesla or more. The main magnetic field B0 is approximately homogeneousin the field of view. The main magnetic field B0 may extend throughoutthe RF cabin 12. Different regions within the RF cabin 12 may besubjected to stronger or weaker magnetic fields. For example, themagnetic field B0 may be weaker in regions near the ends of the mainmagnet 32, but such regions may remain sufficiently within the magneticfield to give rise to challenges for integrating the PET subsystemcomponents into the RF cabin.

The MR subsystem uses the strength of the main magnetic field B0 toalign the nuclear magnetization (i.e., spins) of atomic nuclei in thesubject (e.g., hydrogen atoms in water). High-field systems (e.g., 1.5 Tor 3 T and more) may be used to improve the signal-to-noise ratio. Radiofrequency (RF) fields are used to systematically alter the alignment ofthis magnetization. The nuclear spins of atomic nuclei of the patientare excited via magnetic RF excitation pulses that are transmitted viaan RF antenna, shown in FIG. 1 in simplified form as the body coil 36,and/or possibly a local coil arrangement. The RF excitation pulses aregenerated, for example, by a pulse generation unit controlled by a pulsesequence control unit. After being amplified using an RF amplifier, theRF excitation pulses are routed to the body coil 36 and/or local coils.The excitation causes the nuclei to produce a rotating magnetic fielddetectable by the MR scanner.

The response to the RF excitation pulses may be manipulated byadditional magnetic fields applied by the gradient coils 34 to build upenough information to construct an image of the body. The gradient coils34 radiate magnetic gradient fields in the course of a measurement inorder to produce selective layer excitation and for spatial encoding ofthe measurement signal. The gradient coils 34 are controlled by agradient coil control unit that, like the pulse generation unit, isconnected to the pulse sequence control unit.

The signals emitted by the excited nuclear spins are received by thebody coil 36 and/or at least one local coil arrangement. The body coil36 may have a one-piece construction or include multiple coils. Thesignals are in a given frequency band. For example, the MR frequency fora 3 Tesla system using the spin of hydrogen is about 123 MHz+/−350 KHz.Different center frequencies and/or bandwidths may be used. The voltageinduced in the body coil 36 may be amplified by a low-noise preamplifier(e.g., LNA, preamp) and forwarded to receive electronics. A switchingarray (e.g., BCCS) may be installed between the receive antennas and thereceivers. The switching array routes the currently active receivechannels (e.g., the receive channels currently lying in the field ofview of the magnet) to the receivers present. More coil elements maythen be connected than the number of receivers present to allow, in thecase of whole-body coverage, only the coils located in the field of viewor in the homogeneity volume of the magnet to be read out.

In some MR tomography procedures, images having a high signal-to-noiseratio (SNR) may be recorded using local coil arrangements (e.g., loops,local coils). The local coil arrangements (e.g., antenna systems) aredisposed in the immediate vicinity of the examination subject on(anterior) or under (posterior) or in the patient. The received signalsare amplified by associated radio-frequency preamplifiers, transmittedin analog or digitized form, and processed further and digitized by areceiving unit. The recorded measured data is stored in digitized formas complex numeric values in a k-space matrix. An associated MR image ofthe examination subject may be reconstructed using a multidimensionalFourier transform from the k-space matrix populated with values. For acoil that may be operated both in transmit and in receive mode, such asthe body coil 36 and/or the local coil, correct signal forwarding iscontrolled using an upstream-connected duplexer.

From the measured data, an image processing unit in the MR subsystemequipment 14 generates an image. The image is displayed to a user viathe operator console 20 and/or stored in a memory unit. A centralcomputer unit in the MR subsystem equipment cabinet may control theindividual system components.

Combinations of medical imaging techniques, so-called “hybridmodalities,” may provide a high local resolution modality (e.g., MRimaging) with a modality with high sensitivity (e.g., PET). Bothdetailed anatomy and functional information may be provided in spatialalignment and without errors introduced by temporal discontinuity. Whilethe MR-PET combination may present imaging advantages, simultaneousoperation of PET and MR subsystems also presents an interoperabilityrisk if the electronic and electromagnetic aspects of the subsystemsinterfere with one another. For example, performance of the MR subsystemmay be detrimentally affected if the PET subsystem performs digitalsampling and other data processing at a location or in a manner wherethose activities interfere with the MR receivers at the spin frequenciesof the MR subsystem, e.g., 123.212 MHz for 3 T Hydrogen. For example, RFsignals (either directly, or mixed with other signals) from the clocksused in PET sampling and/or data processing may interfere with theoperation of the MR receivers.

Notwithstanding these risks, the design architecture of the system 10places PET sampling (or digitization) and/or other electronics insidethe RF cabin 12 to be near the PET detectors 30. The PET detectors 30are positioned inside of the magnet 32, e.g., within the magnet bore orother opening. The PET detectors 30 are arranged individually or ingroups. The location and configuration of the PET detectors 30 relativeto the MR components inside of the magnet 32 may vary. Interference inthe signal chain may be introduced by this positioning. By being withinthe RF cabin 12, the PET detectors 30 are within the magnetic fieldgenerated by the magnet 32. Being within the core of the magnet 32, thePET detectors 30 are subjected to similar B0 magnetic field strength anduniformity as the patient. With the PET sampling and other dataprocessing components inside the RF cabin 12, the interconnect distancebetween the PET detectors and PET digital sampling and data processingcomponents is minimized or reduced as compared to placement of theprocessing components outside the RF cabin 12. This positioning of thedata processing components of the PET subsystem within the RF cabin 12removes a potential source of signal distortion or loss for the PETsubsystem.

The digitization electronics of the PET subsystem include an array 40 ofdata processing units 42 of the system 10 disposed in the RF cabin 12.Each processing unit 42 may be configured as, or include, a dataacquisition unit (DAU) configured to acquire the PET detector signalsfor digitization and other processing. Each DAU 42 in the array (DAU₁through DAU_(n)) 40 may be disposed within a discrete housing or otherenclosure separate from the other units 42 as well as from the scanningcomponents inside the RF cabin 12. As described below, it may be usefulto dispose the DAU array 40 near the PET detectors 30 to minimize signaldistortion or other loss. The DAU array 40 may be mounted on, adjacentto, or otherwise proximate to the scanning components of the system 10.The DAU array 40 may alternatively or additionally be spaced from thescanning components. In some cases, the spacing may be adjustable. Inother alternative embodiments, a single housing is provided for all ofthe DAUs 42 or for one DAU.

The heat generated from the DAU array 40 is removed from the RF cabin 12via a cooling system 44 in communication with one or more coolingdistribution networks 46. The cooling system 44 may include a pump fordistributing water or other coolant through pipes of the distributionnetwork 46. The distribution network 46 may include inlet and outletmanifolds. The coolant may be distributed to one or more coolinginterfaces 48 in thermal communication with the DAU array 40, such asthrough metallic blocks or a block in contact with the housings of theDAU array 40. The cooling system 44 may provide any one or more fluidsother than water (e.g., air) to remove heat from the RF cabin 12.

Connections for power and data communications with the DAU array 40 areprovided via a filter plate 50 disposed along a wall or other boundaryof the RF cabin 12. The filter plate 50 may include a set of RF-tightinterfaces that allow power and data connections to enter the RF cabin12 with little introduction of noise. Each interface may include afilter configured to remove from each power line any frequencycomponents that would interfere with the operation of the MR scanner. Inthis example, a respective power connection and, thus a respectivefilter, is provided for each DAU 42 in the array 40. The filter plate 50may include a separate interface (or set of interfaces) for the data andcontrol signals for the DAU array 40, which may be carried via fiberoptic links. The optical signals carried by the fiber optic links may bemultiplexed either inside or outside the RF cabin 12. The RF filterplate 50 may include any one or more interfaces for other signals,including, for instance, a clock signal. The configuration,construction, and other characteristics of the filter plate 50 may vary.

The electronics of the DAU array 40 are powered by DC power provided viaa power supply system 52. The power supply system 52 is located outsideof the RF cabin 12, and is coupled to the RF filter plate 50 to providethe DC and other power via the filters to the scanning components in theRF cabin 12. The DC power may be developed from single-phase line power(e.g., 230 Volts). The power supply system 52 may be modular. In oneexample, separate DC power signals are developed for each filter. Analternative embodiment may include power distribution equipment insidethe RF cabin 12, such that the filtering provided by the filter plate 50may only act upon a single DC power line. Yet another alternativeembodiment may integrate one or more components of the power supplysystem 52 with the PET subsystem equipment 16.

FIG. 2 shows the PET subsystem components in the RF cabin in greaterdetail. Each DAU 42 may be configured to support the processing of thesignals from the PET detectors 30 within the RF cabin. Each DAU 42 mayinclude (1) an interface for one or multiple PET block detectorcassettes, (2) a thermally conductive interface for heat removal, (3) afiltered power interface, and (4) optical and/or RF interfaces for clockdistribution, command and control communications, and communications forevent and general purpose data output. Each of these and other aspectsof the DAU 42 may allow the PET subsystem to process the signals withlittle to no detrimental effect on the operation of the MR subsystem.

The PET detectors 30 include scintillation crystals or other radiationdetectors that may be arranged in cassettes 60 of, for instance, six oreight detector blocks 62. The detector cassettes 60 are arranged in aring, arc, or otherwise partly around the tubular bore region or otheropening for scanning the patient. For example, each cassette 60 ispositioned such that the detector blocks 62, which may be elongate tosupport whole body imaging and/or maximize the PET field of view, arelinearly positioned or oriented along the long or main axis of thesubject bore. Any number of cassettes 60 (e.g., 48, 54, 56, etc.) may bespaced around the subject bore. In one embodiment, each detector block62 includes a crystal array (e.g., a 12×12 array of 2.5 mm crystals).The photons generated by each crystal array are captured by a number ofphotodiodes in each detector block 62. The photodiodes may be avalanchephotodiodes (APDs). For example, nine APDs may be provided for eachblock 62. In other embodiments, the scintillation crystals are coupledto photomultiplier tubes. The scintillation crystals may include bismuthgermanium oxide, gadolinium oxyorthosilicate, or lutetiumoxyorthosilicate crystals, but other crystals may be used.

Each detector block 62 generates three output signals, two positionsignals and one energy signal. These signals are analog signals in APDembodiments. In other embodiments, each detector block 62 may includedigital photodiodes. Each of the signals may be provided as adifferential signal pair output. In an example with 56 APD-baseddetector cassettes 60, each having eight detector blocks 62, a total of1,344 differential signal pairs are presented by the set of detectorcassettes 60. Coaxial, twisted pair, or other cables 64 may be used tocarry the analog signals from the detector blocks 62 to the DAU array40. In one example, each cable 64 that connects one of the detectorcassettes 60 with one of the DAUs 42 includes 24 signal pairs to carrythe analog output signals. Alternatively or additionally, signalencoding may be used to carry the analog signals in the cables 64. EachDAU 42 may handle one or more detector cassettes 60. In this embodiment,a pair of detector cassettes 60 are coupled to a respective one of theDAUs 42. Each DAU 42 includes a corresponding number of inputtermination or other ports 66 to which the cassette cables 64 areattached. Each input port 66 provides an interface for each detectorgroup of signal pairs provided by a respective one of the detectorcassettes 60. The input port 66 may be configured to provide an RF tightconnection and balanced interface for the differential signal pairs(e.g., 16 position and eight energy), as well as for the filtered, splitpower to the detector blocks 62.

Each input port 66 may be coupled to one or more circuits configured toprovide front-end filtering of the cassette detector signal inputs. Thecircuits may include, for instance, one or more band-pass and/orlow-pass filters to select the frequency range of the detector signalsand block any other frequencies. For example, a pole filter may be usedto block the RF excitation frequency of the MR subsystem (e.g., 123MHz). Other filter topologies (e.g., pole or notch filters incombination with other filters) may be used. The detector signals mayhave a relatively wide bandwidth (e.g., 40-60 MHz). The DAU 42 includesa number of other circuits shown schematically at 67 for amplification,analog-to-digital conversion, and/or other signal processing. Thecircuits 67 are configured such that each PET detector event isquantized and time stamped by one of the DAUs 42.

Each DAU 42 may have a dedicated cooling interface 68 to remove the heatgenerated by the circuits 66. The cooling interface 68 may include aportion of the housing of the DAU 42, or include a chill plate, frame orother structure in thermal communication with the DAU housing. In oneembodiment, the cooling interface 68 includes a metal platform (e.g.,aluminum, copper, or any combination thereof). The metal platform orother cooling interface 68 may be coupled to the cooling system 44(FIG. 1) or other device or system for access to, or other communicationwith, a cooling fluid. A variety of different coolant fluids may beused. For example, the cooling interface 68 may be water-based.Alternatively or additionally, the interface 68 may include a heat sinkin thermal communication with the DAU 42. The heat sink may beconfigured to be exposed to forced air or other fluids.

The DAU 42 may also include one or more thermally conductive paths 70between the thermal interface 68 and internal DAU heat sources. Thethermally conductive paths 70 may include components external to the DAUhousing, as well as or alternatively components within the DAU housing.The thermally conductive paths 70 may include electrically insulatinggaskets or other spacers via which the DAU 42 is mounted to the coolinginterface 68. The thermally conductive paths 70 may be made of thermallyconductive polymer materials. In one embodiment, the thermallyconductive gaskets have a thickness of about 2 mm, and are made ofsilicone. The materials, construction, configuration, location, andother characteristics of the thermally conductive paths 70 may vary.

The DAU 42 also includes an input termination or other port 72 for a DCpower supply. The input termination 72 may include or be coupled to oneor more power filters to remove any noise that may have been introducedbetween the RF filter plate and the DAU 42. The filtering may includeone or more low-pass or notch filters. The filter(s) may be configuredto block signals at the RF excitation frequency of the MR subsystem. Thefilter topology may vary. The filtering may be provided in addition tosimilar filtering provided at the filter plate 50 by a power supplyfilter 73.

The DAU 42 may include a number of other ports configured to provideinput/output (I/O) interfaces. An I/O port 74 is configured to receive aclock signal, and an I/O port 76 is configured to support communicationswith the remainder of the PET subsystem. The clock signal port 74 mayinclude a coaxial connector for receiving a sine-wave clock, which maybe referenced to chassis ground (e.g., chassis ground of the PET filterplate described herein). The clock signal may be used as a system clockfor the PET detector data sampling and other signal processingimplemented by the DAU 42. Alternatively or additionally, a system clockmay be developed from a timing signal provided via the I/O port 74. Forexample, the clock signal received via the I/O port 74 may be a sine orsquare wave, which is then used to develop a conventional square waveclock once inside the RF tight housing of the DAU 42. Transmission ofthe sine wave along one or more cables 77 inside the RF cabin 12 may beless of a risk to the operation of the MR subsystem than thetransmission of a conventional square-wave clock. The I/O port 74 mayinclude multiple connections to support both reception and transmissionof the clock signal, which may be useful for distributing the clocksignal to the entire DAU array 40 without losses or other variance.

The I/O port 76 may include an optical fiber connector to support fiberoptic communication signals over a fiber optic link 78. The opticalsignals may include both the digitized PET detector signal data as wellas control signals and other communications between the PET subsystemequipment 14 (FIG. 1) and the DAU 42. The optical fiber connector of theI/O port 76 may be configured to limit or prevent RF leaks into or outof the DAU 42.

The DAU 42 includes one or more RF shielded compartments to limit EMIfrom entering or exiting the DAU 42. Because the DAU 42 is locatedwithin the RF cabin, noise from the DAU 42 should be reduced to aminimum, which is challenging due to the high channel count of the PETsubsystem (e.g., 1344 signal pairs coming from the PET detectors). TheRF shielding of each DAU 42 achieves high levels of bi-directionalattenuation at the MR frequency, for both differential and common modesignals, thereby preventing interference with the MR imaging. Withsufficient attenuation, both MR and PET scans may be performed at a sametime. The DAU 42 may include several components that, without shielding,may emit electromagnetic radiation at the MR frequency. These componentsmay include front end filters and amplifiers, analog-to-digital (A/D)converters, and digital signal processing circuitry. The shieldedcompartments also protect these components from the gradient and RFfields of the MR subsystem. Such protection is in addition to theprotection provided by a number of filters within the DAU 42 (e.g.,power supply filters and clock input/output (I/O) filters).

Each DAU 42 may include a housing 80 that fully encloses its electroniccomponents for RF tight shielding. The housing 80 may be made of one ormore metals (e.g., copper) having a thickness corresponding with, forexample, five skin depths at 20 kHz. The housing 80 may include anynumber of partitions or walls that define separate compartments forvarious electronic components. The partitions or walls may also have asimilar thickness. Alternatively, a single compartment is formed by thehousing 80.

FIG. 3 shows one example arrangement of an array 82 of DAUs 84 in the RFcabin. The DAU array 82 is disposed along an MR scanner 86, which may beconsidered a hybrid or integrated MR/PET scanner because PET detectorsare disposed within a bore or opening 88 defined by a magnet of the MRscanner 86. In this example, the DAU array 82 is disposed along a planarend face 90 of an enclosure or other housing 92 of the MR scanner 86.The end face 90 may correspond with a back side of the MR scanner 86opposite the side through which the patient table enters the MR bore 88.The enclosure 92 may include a main section 94 configured to cover themagnet 32, the gradient coils 34, the body coil 36 and other scanningcomponents of the MR subsystem (see FIG. 1). The main section 94 has atubular shape that matches the shape of the magnet 32 of thisembodiment. The end face 90 may lie between a pair of annular rims(e.g., shaped as concentric circles). The end face 90 may have adisc-shaped or annular surface on which the array 82 is mounted or alongwhich the array 82 is disposed. The main section 94 and other portionsof the enclosure 92 may be shaped differently to match other magnetshapes. The enclosure 92 includes a base section 96 to cover a stand orother support structure, as well as an auxiliary cabinet section 98 toenclose electronics, user interface, or other auxiliary or controlelements of the MR subsystem. The auxiliary cabinet section 98 may belocated alongside the main section 94 and oriented along a main axis ofthe MR scanner 86 (or magnet).

The DAU array 82 is oriented in a plane parallel to the end face 90.Taken together as a group within that plane, the DAUs 84 form a disc- orstar-shaped pattern or cluster disposed around the bore 88. Each DAU 84is disposed along a respective radial line extending outward from thecenter of the bore 88. Each DAU 84 includes an RF shield housing 100tapered to form a sector of the disc shape. The RF shield housing 100includes an inner end 102, an outer end 104, and a pair of tapered,lateral sides 106. In this example, the lateral sides 106 are oppositesides and extend along radial or sector lines of the disc shape. Thetapering leads to the inner end 102 being shorter than the outer end104, as in a pie slice shape. Each DAU 84 may be disposed adjacent apair of neighboring DAUs 84 with minimal spacing along the radial lengthof the DAU 84, allowing a large number (e.g., 28) of DAUs to fitalongside one another as a contiguous DAU array. The number of DAUs 84in the array 82 may vary based on the number of PET detectors.

The disc- or star-shaped DAU packaging arrangement shown in FIG. 3 isconfigured to minimize interaction between the PET and MR subsystems.The arrangement may reduce variations in the B0 and B1 magnetic fieldsof the MR subsystem that might otherwise result from the noise generatedby the data processing in the DAUs 84. The DAUs 84 are disposed in asymmetrical arrangement that may help reduce or minimize EMI or EMC. Inthis example, the symmetry is radial, or relative to the center of thebore 88, as well as axial due to the planar alignment of the DAUs 84.Both symmetrical aspects of the packaging arrangement are directed tominimizing or preventing any impact on MR image quality. PET signalintegrity is also maximized via this symmetrical DAU placement, suchthat multiple potential interoperability issues between the MR and PETsubsystems may be addressed. In alternative embodiments, the DAUs may bearranged in a half-disc or half-star configuration, or in any otherconfiguration in which the DAUs are arranged along radial lines relativeto the MR bore.

The disc- or star-shaped arrangement of the DAU array 82 does not alterthe form factor of the MR scanner 86 or substantially change thedimensions of the MR scanner 86. The MR scanner 86 may be shaped andsized in accordance with strict siting conditions. The installation siteof the MR scanner 86 may present restrictions on the overall length,width, and height of the unit. These restrictions may arise from thesize of RF cabin doors or other installation access points. For example,the overall length of the unit (i.e., the dimension along the main axisof the bore 88) may be limited to two meters. The DAU array 82 addslittle to that length, because each DAU 84 is oriented along the same,upright plane adjacent the MR scanner 86 and because each DAU 84 has aflattened profile. The lateral dimensions of each housing 100 exceed theheight (or thickness relative to the main axis of the bore 88) of thehousing 100, where the lateral dimensions correspond with the dimensionsaligned with the upright plane along which each DAU 84 is disposed. TheDAUs 84 also may not extend radially beyond the end face 90 of the MRscanner 86. The RF shield housing 100 of each DAU 84 may have a radiallength that corresponds roughly with the radial extent of the annularsurface of the end face 90. The DAUs 84 avoid blocking the opening ofthe bore 88 by not extending beyond an inner rim of the end face 90, andallow the MR scanner 86 to remain mounted on a standard supportstructure in the base section 96 by not extending beyond an outer rim ofthe end face 90.

In this arrangement, each DAU 84 in the array 82 services a pair of PETdetector (e.g., avalanche photo diode) cassettes located within the MRbore 88 in front of the gradient coil and behind the body coil RF screen(FIG. 1). A pair of PET detector cables 108 terminate at a connectorlocated at the inner end 102 of the DAU 84. The disc- or star-shapedarrangement minimizes the length of each cable 108, insofar as the innerend 102 is located adjacent the edge of the bore 88. The minimal lengthof the cables 108, in turn, minimizes the distortion of the analogsignals developed by the avalanche photodiodes before the analog signalsare digitized in the DAUs 84. The DAU packaging arrangement also allowseach cable 108 to have the same length. The equal lengths and evenlyspaced radial positioning of the cables 108 provide additional radialsymmetry to minimize EMI and EMC.

Another connector 110 is located at the outer end 104 of the DAU 84 forthe termination of a dedicated power cable for each DAU 84. In thisexample, separate connectors are provided for the clock, control, anddata signals, although a common connector may be used. A connector 112is located at the outer end 104 for the clock signal, and a connector114 is located on a front face 116 of the housing 100 for the fiberoptic link carrying the incoming control information and outgoing PETdata. The connector 114 may include a slotted block of, for example,metal, configured as a socket or other opening to receive ends of thefiber optic link.

All of the above-described connections may be configured to presentradially symmetrical interconnect cabling leading to and from the DAUs84. The symmetry of the interconnect cabling is relative to the MRmagnet bore 88. Such symmetry minimizes or reduces EMI and EMC betweenthe MR and PET subsystems. EMI and EMC may be further reduced by routingthe interconnect cabling through one or more cable guides 118 disposedalong the outer rim of the array 82, which, in this case, correspondswith the outer rim of the end face 90 of the enclosure 86. Each cableguide 118 may be configured as an elongate bracket having a curvaturethat matches the curvature of the outer rim. In this example, the cableguide 118 may be an L-bracket, but a variety of other configurations maybe used.

Performance of the PET subsystem is improved by locating the DAUs 84near the PET block detectors (FIGS. 1 and 2). The number of DAUs 84 mayvary from the example shown. Spreading the DAUs 84 around the radialextent of the MR housing may also provide advantages in keeping thecabling, RF and thermal densities to moderate levels. The packagingarrangement is thus scalable. The system is also scalable because of themodularity of the DAU array 82. Manufacturability and serviceability mayalso be enhanced by the modularity of the DAU array 82. For example, theDAU array 82 may be partitioned to diagnose a problem.

FIG. 4 shows how the location of the DAU array 82 in the RF cabin may beoptimized or adjusted relative to the MR scanner 86. The location of theDAU array 82 may be adjusted to optimize a spacing between the DAU array82 and the MR subsystem. Adjusting that spacing may allow levels of EMIor EMC to be reduced, which may be useful in connection with MRsubsystems having high field strengths or fringe fields. For instance,the fringe field may vary between MR subsystems based on differences inthe type, strength, or depth (i.e., axial length) of MR magnet or bore.The DAU array 82 may be movable in the axial direction (i.e., thedirection of the main axis of the MR magnet) to introduce a gap 120between the end face 90 of the enclosure 92 and a support framework orother structure on which the DAUs 84 are mounted.

The support framework may include a base (not shown) on which the array82 rests. The construction, configuration and other characteristics ofthe base and the support framework may vary considerably. The base maybe used to position the array 82 at a desired distance from the MRmagnet and/or other components of the MR subsystem.

The disc- or star-shaped arrangement of the DAU array 82 and themounting of the array 82 on a base allows the gap 120 to be optimized.The size of the gap 120 may be adjusted to address differences in thedepth of the magnet bore. Examples of gap sizes include embodiments inwhich the gap size is less than or roughly equal to the axial thickness(or height) of each DAU 84, e.g., about 4 inches or less, andembodiments in which the gap is longer than the axial thickness of theDAU 84, e.g., from about 8 to about 10 inches. The gap size may rangefrom zero to any desired spacing.

The relative positioning of the DAU array 82 relative to the MR scanner86 (e.g., away from the scanner 86 in the axial direction) may be usefulto mitigate Lorenz forces and/or B0/B1 homogeneity degradation. Thetime-varying magnetic fields from the MR gradients induce currents inany metal structures of the DAU array 82 (e.g., each DAU housing),causing the metal structures to vibrate, which, in turn, createsmagnetic fields that may oppose or otherwise degrade the MR fields.Increasing the size of the gap reduces the Lorenz forces to reduce suchdegradation. In alternative embodiments, the DAU array 82 is fixed tothe MR scanner 86 without an assembly to adjust any gap.

FIG. 5 shows a schematic view of one of the DAUs 84 in an exampleembodiment having the tapered housing 100, but non-tapered housings maybe used. Near the inner end 102, the DAU 84 includes an interface board136 to support the pair of connections with the PET detector cassettecabling. The interface board 136 receives the cassette cabling and mayprovide an interconnect to both the detector signal and the powerfilters. The interface board 136 may be passive. The interface board 136is part of an analog section of the DAU 84 that also includes a numberof filters to remove the noise generated by the RF excitations of the MRsubsystem (e.g., 123 MHz). The interface board 136 and other componentsof the analog section of the DAU 84 may be disposed in one of multiple,separate compartments of the housing 100. The filters may be arranged ina pair of cassette filter sets 138A, 138B, one for each of the detectorcassettes serviced by the DAU 84. Each set 138A, 138B may include one ormore power supply output filters 140 and one or more detector signalfilters 142. The detector signal filters 142 generally block the RFexcitation frequencies from reaching the downstream components in theDAU 84, such as the analog-to-digital converters. The detector signalfilters 142 also prevent any noise from the DAU 84 at those frequenciesfrom interfering with the operation of the MR subsystem. The powersupply output filters 140 provide similar functionality with respect tothe DC power delivered to the PET detectors 30 (FIG. 1). Power deliveredto the DAU 84 is filtered by a power input filter 144, which may bedisposed on a dedicated board 145. The power input filter 144 may bedisposed in a discrete housing adjacent to the housing 100, or bedisposed in a dedicated compartment of the housing 100. The analogsection of the DAU 84 also includes a pair of buffer amplifiers 146,which may be configured to drive further amplifiers (e.g., flashamplifiers) in the digital section of the DAU 84. Further filtering atthe MR RF frequency may occur at the interface between the analog anddigital sections of the DAU 84.

The digital section of the DAU 84 may include a number of discrete unitsfor processing the signals from respective sets of PET detector blocksseparately. In this example, the digital section includes signalprocessing quadrants 148, each of which may handle a number of detectorblocks (e.g., four detector blocks). Each signal processing quadrant 148may include analog-to-digital converter circuitry, such as feedbackamplifiers and other devices, for sampling or otherwise digitizing theanalog detector signals. The digital section of the DAU 84 also includesan I/O interface 150 at the outer end 104 of the DAU 84, which may beconfigured for dual-fiber optical communications with other componentsof the PET subsystem. The optical communications may include receptionof control signals and transmission of digital data representative ofthe PET detector signals. The digital section of the DAU 84 may beenclosed in one of the separate compartments of the DAU housing 100.

Each compartment of the DAU housing 100 may be RF shielded from theother compartments via a wall or other divider. The walls or dividersmay be aligned with one or more grounding traces on a circuit board ofthe DAU 84 to isolate the compartments from one another. Thecompartments need not correspond with the lines shown in FIG. 5. Thewalls or dividers defining the compartments may be constructed in thesame manner as the external walls of the housing 100. One or more of thewalls or dividers may, in fact, correspond with external walls of thehousing 100. As shown in the example of FIG. 6, the housing 100 includesa compartment 152 disposed near the inner end 102. The compartment 152is separated from a main compartment 154 of the housing 100 via a notch156 defined by a pair of walls 158, 160. In this example, thecompartment 152 encloses circuitry 162 disposed on the cassetteinterface board 136. The components of the DAU 84 disposed in separatecompartments may vary, including, for example, the location of anyexternal walls of the housing 100 that act as dividers.

FIG. 6 also shows one example of a cooling interface 164 for thermalmanagement of each DAU 84. The cooling interface 164 includes an area inwhich the housing 100 is adjacent to a thermally conductive structure166. Cooling pipes or other conduits 168 are in thermal communicationwith the structure 166, which, in turn, is in communication with thehousing 100 via the interface 164. Heat generated by the circuitry inthe DAU 84 is dissipated via the fluid passing through the cooling pipes168. Each cooling pipe 168 may run through or adjacent to each of theDAU housings 100. In some embodiments, including those having thetapered DAU housings forming the disc-shaped arrangement, the coolingpipes 168 may be shaped and disposed as concentric rings.

In one embodiment, the thermally conductive structure 166 includes ametal platform on which the housing 100 rests. For example, the metalplatform may be made of aluminum and copper. The platform includes aflat, top side 170 that defines the interface 164 and a bottom side 172opposite the top side 170. The platform includes a number of sidewalls174 between the top and bottom sides 170, 172. The bottom side 172 maybe open or partially open to allow air cooling of the interface 164.Forced air cooling may be provided as an alternative, or in addition, tothe cooling provided by the pipes 168.

FIG. 7 shows an alternative integrated MR/PET scanner 174 that providesa packaging arrangement for an array 176 of a DAUs 178 within the RFcabin. The DAU array 176 is located along a side 180 of a housing orenclosure 182 of the scanner 174. The location on the side 180 differsfrom the location along an end face of the above-described disc- orstar-shaped DAU arrangement. For instance, the end face has the MR boreor patient opening. In this embodiment, the DAU array 176 may be locatedalong one or more of the sides or surfaces of the housing 182 not havinga magnet bore or opening. The side 180 corresponds with a lateral orlongitudinal surface of the housing 182 disposed along an axial lengthof the MR scanner. The side 180 is disposed between the faces having amagnet bore or other opening. In this example, the side 180 is a curvedsurface. The curvature may match that of the MR magnet enclosed by thehousing 182. However, the housing 182 need not have a shape thatcorresponds with the MR magnet of the scanner 174.

The lateral location of the DAU array 176 along the side 180 may beuseful from both a static magnetic field magnitude standpoint (e.g., theB0 field) and a dynamic, or time varying, field standpoint (e.g., thegradient and B1 fields). The DAU array 176 may be subjected todiminished fringe field strengths in the lateral region along the side180. The extent to which degradation of the B0 and B1 fields may occuris reduced. Also, less noise at the MR frequency (e.g., 123 MHz) may beintroduced into the I/O signals and power cabling for each DAU 178 withthe array 176 at the lateral side location.

The DAU array 176 may be distributed among multiple side locations. TheDAU array 176 need not be located along a single lateral side. Forexample, one-half of the DAUs 178 may be disposed along one lateralside, while the other half of the DAUs 178 are disposed on the oppositelateral side. The axial positioning of the two sets of DAUs 178 may bematched to provide axial symmetry relative to the MR magnet and othersubsystem components.

Whether disposed on one or more lateral sides, the DAUs 178 of the array176 may be disposed in a rack pattern or arrangement that may supportsymmetries relative to the MR subsystem or MR fields. The DAUs 178 ofthe array 176 are stacked in one or more columns or other groupings. Inthis example, the DAUs 178 are stacked in a rack pattern having threecolumns disposed in a row oriented in parallel with the main axis of theMR magnet. The columns may have an equal height (e.g., an equal numberof DAUs 178) to provide one or more degrees of symmetry relative to theMR fields. Further symmetries are provided by equal spacing between thecolumns and equal spacing between individual DAUs 178 in each column.The rack pattern need not dispose the DAUs 178 in vertical stacks. Forexample, the DAUs 178 may be stacked or mounted in a non-vertical mannerto, for instance, track the curvature of the housing 182. The number ofDAUs 178 in each column may vary such that columns may have an unequalnumber of DAUs 178.

The DAUs 178 may be supported by shelves, mounting brackets, or otherstructures projecting from a back panel or other riser. The DAUs 178 maybe grounded to the MR magnet in one or more groups. The supportstructure and collective grounding notwithstanding, the DAUs 178 withinthe rack-mounting arrangement may be galvanically isolated from oneanother as described further below. The support structures may include,define, or support a respective cooling interface for each DAU 178. Oneexample cooling interface uses the spacing between the DAUs 178 as aflow path for forced air or other cooling fluid(s).

Each DAU 178 need not have a box-shaped housing 182 as shown in FIG. 7.The lateral side location of the DAU array 176 is not limited to anyhousing shape, profile, or form factor. The DAUs 178 need not be stackedin the orientation shown. For example, each DAU 178 may be rotated to amore vertical or upright orientation with each DAU 178 disposed on anend adjacent to an end of a neighboring DAU 178. In such an orientation,the larger faces of all of the DAU housings 182 are aligned in a pair ofplanes.

As with the above-described embodiments, the DAU array 176 presents amodular architecture for the processing of the PET detector signals.Each DAU 178 may receive signals from one or more PET detectors. In thisexample, each DAU 178 may service two detector cassettes. The DAU array176 may thus support a large number (e.g., 56 or 60) of detectorcassettes.

The rack pattern of the DAU array 176 may also provide a symmetricalarrangement of PET detector cabling. Cables 184 extend from the DAUs 178to the bore or opening as shown schematically in FIG. 7. In thisexample, an upper half of the cables 184 may connect to an upper half ofthe PET detectors, while a lower half of the cables 184 may connect to alower half of the PET detectors. Such interconnect cabling may besymmetrical about a horizontal line segment passing through the centerof the bore. Further symmetry about the MR bore (or the line segmentpassing through the bore) may be achieved by equalizing the lengths ofthe cables 184. For those cables 178 that enter the bore nearest to theDAU rack, the cables 184 may be deflected upward (or downward) along aportion 186 to introduce extra length, and may run to the DAUs 178 inthe column farthest away from the bore. Even with the added length, theoverall length of each cable 184 is still relatively moderate, and shortin comparison with systems in which the analog detector signals arecarried outside of the RF cabin. These symmetries may be useful inconnection with both closed and open MR subsystems.

Heat generated by the DAU array 176 may be removed by a coolinginterface provided with water or other fluid transported via pipes 188,190, which terminate at a manifold 192. Tubing may then carry the fluidalong paths in thermal communication with the DAUs 178. One example ofsuch tubing is shown in connection with another manifold 194 thatprovides fluid to cool the PET detectors in the MR bore. The manifold194 supports a set of tubes 196 to deliver chilled fluid to the PETdetectors and a set of tubes 198 that return the fluid after exposure tothe heat generated by the PET detectors.

The disclosed systems may include a power distribution system to supportthe location of the PET subsystem electronics of the DAUs in the RFcabin. The power distribution system may include the filter plate 50 andthe modular power supply system 52 shown in FIG. 1. Further detailsregarding examples of these power distribution components are presentedin connection with FIGS. 8 and 9.

The power distribution system may be configured to supply power to thePET subsystem components in the RF cabin while keeping the powerspectral density of the PET electronics (e.g., the above-describeddigital sampling and processing circuitry) controlled and out of the MRreceiver frequency bands (e.g., 123.212 MHz+/−500 KHz for 3 T MRsubsystems). This aspect of the power distribution system may bechallenging in view of the length of the power cabling (e.g., up toabout 15 meters), and in view of the current magnitudes involved (e.g.,a total of about 850 A to the DAUs and the PET detectors). In oneexample, the power distribution system supplies approximately 4 kW ofpower to the PET subsystem components in the RF cabin. The powerdistribution system also distributes multiple classes of power to eachDAU and detector cassette in a way that minimizes the EMI/EMCsusceptibility of the PET subsystem to the MR gradient switch-modenoise, gradient fields, and RF bursts.

The power distribution system may include separate power sources for theMR subsystem. For example, a Verio 3 T gradient amplifier may be used tosupply up to 4 kV at 1000 A peak to each of three gradient coils. The MRBody coil may be driven by an RF power amplifier capable of delivering35 KW of power. Without more, EMI from these MR power sources can becoupled into the PET power cables.

The power distribution system may support PET subsystem digital samplingand digital signal processing on the large scales described above withinthe MR RF cabin via a number of isolated, floating, modular powersupplies. Each power supply may drive a respective DAU though anindividual filter module interconnected with shielded twisted paircabling such that power is distributed in a star configuration to theDAU/PET detector load.

FIG. 8 shows one example of a filter plate 200 of the power distributionsystem. The filter plate 200 may be mounted to define RF tight access tothe RF cabin. The filter plate 200 includes a respective power filter202 for each DAU in the DAU array. This modular approach may be extendedto the power supplies outside of the RF cabin, as described below. Eachpower filter 200 may thus be associated with a respective power supplymodule/DAU pair. Each power filter 200 may be configured to remove orblock any frequencies that may disturb the MR subsystem, including thosein the MR receiver bands. The filter plate 200 may also include a port204 for the fiber optic communications between the DAUs and the PETsubsystem equipment outside of the RF cabin. The port 204 may include ametal tube 206 or other conductive structure dimensioned and otherwiseconfigured to act as a waveguide filter that blocks frequencies in theRF receiver bands from entering the RF cabin. The aspect ratio of thetube 206 may be selected such that RF noise at those frequencies cannotpass through the tube 206. For example, the tube 206 may have a diameterof in a range from about 2.5 to about 3.0 inches and a length in a rangefrom about 12 inches to about 16 inches. The diameter is selected toprovide ample room for a number of fiber optic cables 208, each of whichmay constitute a bundle of fiber optic links. Each link may have a dualfiber configuration to support both input and output communications fora respective DAU.

The filter plate 200 may also include one or more additional RF tightinterfaces or access ports for a system ground reference (including aground reference mounting location) and one or more clock signal lines.In one embodiment, a single clock signal (e.g., a sine wave) is providedvia the filter plate 200 to a splitter (e.g., 1:30) in the RF cabin. Thefilter plate 200 may alternatively support the transmission of multipleclock lines leading from a splitter or other circuitry located outsideof the RF cabin.

In this embodiment, a respective power cable 210 runs from each powerfilter 202 to a corresponding one of the DAUs in the array. In analternative embodiment, a single power cable may support more than oneDAU by way of a power distribution network within the RF cabin. Eachpower cable 210 also supports the pair or other number of PET detectorcassettes associated with the DAU. The respective power cables 210distribute power in a star configuration. As a result of this and otheraspects of the power distribution system (e.g., modularity), each DAU isgalvanically isolated from the other DAUs. The power cable 210 and othercabling in the power distribution system may be externally shielded andtied to the chassis ground of the filter plate 200. Each power cable 210may include internal power cable conductors such as twisted pair forsingle-ended power, twisted triplet for split rail power, and coax forhigh voltage power.

FIG. 9 shows examples of a power supply rack 220 of the powerdistribution system. The power supply rack 220 is located outside of theRF cabin (i.e., on the other side of the filter plate). The power supplyrack 220 supports a modular approach to providing power to the DAU arrayand PET detectors. The power supply rack 220 includes a number of PETpower supply subsystem (PPSS) assemblies 222. This example has four suchsubsystem assemblies 222. Each subsystem assembly 222 may have a numberof discrete power supply modules (PSM) 224 driven by a power supply hub(PSH) 226. The modules 224 and hubs 226 are mounted or housed in a mainchassis 228, which may have mounting brackets or other interface intowhich the modules 224 and the hubs 226 may be plugged. The entire PETsubsystem within the RF cabin (including, e.g., 28 DAUs) may besupported by four or other number of PPSS assemblies 222. The modularand pluggable nature of the system allows the system to be adjusted orscaled to accommodate different PET subsystems having different DAUarrangements.

Each module (PSM) 224 may run off of AC line voltage (230V) supplied bythe PSH 226. Each hub (PSH) 226 filters the AC line voltage (230V) anddistributes the filtered line voltage to the PSMs 224. Each PSM 224 maygenerate isolated low-voltage power and high-voltage power. The lowvoltage power may be directed to supporting the operation of the DAU andthe PET detectors, while the high voltage power may be directed tosupporting the operation of the PET detectors. Each PSM 224 may deliversuch power outputs through a common-mode power output filter. Each powersupply module may include switch-mode power supplies phase-locked to aPET system clock and/or MR time base. The common-mode power outputfilters of each PSM 224 may help address the considerable common-modenoise that may be generated in a switch-mode power supply scheme.Alternatively, each power supply module includes one or more linearpower supplies.

Each hub (PSH) 226 may include a communications multiplexer using, forexample, the I²C communication protocol, configured to monitor andcontrol each PSM 224 in the subsystem 222. Other communication protocolsmay be used. Each PSM 224 has a companion power filter 202 (FIG. 8)located on the PET filter plate 200 (FIG. 8) mounted on the RF cabin.Respective cabling 230 may couple the filters 202 and the PSMs 224. EachPSH 226 may also provide input line filtering, and include an inputcircuit breaker.

Each PSM 224 may have a circuit breaker 232 or other switch to controlthe operation of the PSM 224. The circuit breaker 232 may cut power tothe entire PSM 224 or otherwise disconnect, shut down, or deactivate thePSM 224. Deactivating or disconnecting one of the PSMs 224 may be usefulin connection with diagnosing an operational problem (e.g., for manualEMI/EMC shielding troubleshooting). For example, if an RF leak isdetected, any number or set of the PSMs 224 may be shut off to determinewhich one(s) of the PSMs 224 may be responsible for the leak. Themodularity of the PSMs 224 and other components of the power supplydistribution system provides a serviceable system. Each PSM 224, powerfilter 202 (FIG. 8), and DAU are scaled up as a group, with each groupisolated from each other group.

The PSMs 224 and the PSHs 226 may be enclosed in EMI-shielded housings.Each housing may have any number of vents for air cooling.

FIGS. 10A and 10B schematically depict an example printed circuit board(PCB) assembly 250 in which partitioning is used to minimize EMIemissions from, and susceptibility in, the DAUs. Geographic partitioningof the PCB assembly 250 may lead to low EMI emissions despite dataprocessing implemented by each DAU. In one example, a number of DAUs(e.g., 28 DAUs) process 1.3 Terra bits of data per second from 1,344differential signals received from the PET detectors coupled to the DAUs(i.e., a total of 2,688 individual signals). Emissions are reduced inthis example despite the large-scale sampling and digitization of thosedifferential signals with 1,344 twelve-bit A/D converters and subsequentprocessing with a corresponding number of high-density fieldprogrammable gate arrays (e.g., 140 FPGAs). Emissions are also reducedvia a configuration of the PCB assembly 250 that integrates EMIshielding into the circuit layouts of each DAU and localizes powerdistribution and management.

The PCB assembly 250 includes a number of PCB layers arranged in astack. A top PCB layer 252 is shown in FIG. 10A, while a bottom PCBlayer 254 is shown in FIG. 10B. Any number of PCB layers may be disposedbetween the top and bottom PCB layers 252, 254 to route signals insupport of various DAU functions. The integrated shielding andgeographic partitioning of the PCB assembly 250 may be provided withinspecific layers and/or between layers. In this example, shielding andpartitioning is provided within a given plane of the PCB layers by anumber of ground partitions. Similar partitioning may be provided inmore than one plane. Further shielding and partitioning is providedvertically (e.g., between or across the stack) in the PCB structure viathe PCB layers themselves. For example, signal transmission may beconfined to interconnects formed in one or more PCB layers near the toplayer 252 (e.g., the top eight layers of the stack). RF currents maythus be confined to small regions or volumes. With no RF signals flowingbeneath the upper subset of the PCB layers, shielding is provided by theremaining PCB layers lying beneath the uppermost layers (such as thosenear the bottom layer 254). The lower subset of the PCB layers may bedirected to power distribution and regulation, as described furtherbelow. Other distribution of functions amongst the layers may beprovided.

A number of ground partitions geographically separate respectiveadjacent circuit regions on the top PCB layer 252 and/or other PCBlayers of the assembly 250. One or more of the ground partitions may becoupled to chassis ground of the DAU by, for example, a seam, wall, orother component of the DAU housing as described herein. The groundpartitions may be used to separate analog circuits from mixed signalcircuits, and/or to separate mixed signal circuits from digitalcircuits.

In the example of FIG. 10A, a ground partition 256 separates a detectorfilter interface 258 from an input buffer amplifier region 260. Thedetector filter interface 258 may be directed to filtering the incomingPET detector signals, the power supplies for the PET detectors, and thei²C communications bus. The detector filter interface 258 may include anumber of circuit boards dedicated to each of these filter functions.The filter boards may be mounted transversely to the top PCB layer 252of the PCB assembly 252 and a PET detector interface board 262, whichmay be independent from the PCB assembly 250. Each filter board may thusstraddle a gap 264 or other divider between the PCB assembly 252 and theinterface board 262. The filter boards may be disposed in parallel toone another. Alternatively, the components on the interface board 262are disposed on an extension of the PCB assembly 250. Those componentsmay then be separated from the detector filter interface 262 by a groundpartition along the line formed by the gap 264. The detector interfaceboard 262 may thus be integrally formed with the top PCB layer 252 as anextension thereof. The PCB layers of the stack may thus be sizeddifferently from one another.

The detector interface board 262 may be configured to couple multiplePET detector cables to the PCB assembly 250 via a number of detectorsignal filters and power supply filters in the detector filter interface258. The detector interface board 262 may couple the PET detectors tothe multiple analog signal filters in the detector filter interface 258.In one example, the detector interface board 262 couples a pair of PETdetector cables with six eight-channel analog signal filters. Thedetector interface board 262 may also be configured to couple the outputpower and i²C filters of the detector filter interface 258. Solder maskalong the perimeters of the upper and lower sides of the detectorinterface board 262 may be removed to reveal a ground partition forcoupling to the DAU housing. Further details regarding the coupling tothe DAU housing via one or more EMI gaskets are set forth below. In oneexample, the solder mask may be removed via an exposure of 100 mils anda finish of emersion silver or gold flash.

The analog circuitry in the input buffer amplifier region 260 may beseparated from the mixed signal circuitry of a signal processing region266 by a ground partition 268. The mixed signal circuitry includescircuitry for analog-to-digital conversion and other PET dataprocessing. For example, the mixed signal processing region 266 mayinclude a number of analog-to-digital (A/D) converters 270 for samplingthe position signals received from the PET detectors. Further A/Dconverters 270 may be provided in the mixed signal processing region 266to sample the energy signals received from the PET detectors. The energyA/D converters in this example are mounted in a pair of geographicallypartitioned regions 272 within the mixed signal processing region 266.The regions 272 are defined by respective ground partitions that may beformed and configured in a manner similar to the above-referenced groundpartitions. One or more other components may be separated from othermixed signal processing circuitry by ground or other partitions withinthe mixed signal processing region 266. For example, the region 266 mayinclude a number of areas 274 where solder mask is removed to expose theunderlying ground plane (e.g., silver) to couple to one or more elementsof the DAU housing, such as one or more walls or seams. The areas 274may thus define local shielding for respective separated regions orcompartments for a corresponding number of circuits, e.g., fieldprogrammable gate arrays (FPGAs).

The ground partitions 274 and, thus, the corresponding FPGAs, may bearranged in separate quadrants or other sections of the mixed signalprocessing region 266 dedicated to specific PET detector blocks (orother groups of PET detector signals). Each quadrant or section mayprocess the PET detector signals from a respective number of PETdetector blocks (e.g., four PET detector blocks). The mixed signalprocessing region 266 may thus be arranged in a PET detectorblock-specific scheme, with the components dedicated to processing PETdetector signals from a certain block (or blocks) located within arespective section of the region 266. This layout of the PET signalprocessing into quadrants or other sections on the PCB assembly 250 mayminimize signal travel during the PET signal processing.

This quadrant- or other PET detector block-based layout may be extendedto other circuit regions (e.g., analog or digital regions) and other PCBlayers of the assembly 250, as described below. For example, the bottomPCB layer 254 (FIG. 10B) may include a region 276 directed to regulatingpower for the mixed signal processing components of the top PCB layer252. The mixed signal processing region 276 may be separated from ananalog region 278 of the bottom PCB layer 254 by the ground partition268. In this example, the mixed signal processing region 276 includes anumber of local linear regulators 279. Each linear regulator 279 may bededicated to a respective quadrant or other section of the mixed signalprocessing region 266. The positioning of the linear regulators 279 onthe bottom PCB layer 254 may not align with the positioning of thequadrant or other section of the mixed signal processing region 276.Each linear regulator 279, while local to a respective quadrant orsection, need not lie directly under the devices or components that theregulator 279 powers in that quadrant or section. An offset in thepositioning of the linear regulators 279 relative to the devices in thecorresponding quadrant or other section may help with thermalmanagement. Alternatively, the positions are not offset.

Power may enter the PCB assembly 250 via a connection or other interfacedisposed within a ground partition 280. A power input filter board (FIG.5) may be coupled to the PCB assembly 250 via the interface. The groundpartition 280 and, thus, the interface for the power input may bepositioned between the input buffer amplifier region 260 and the mixedsignal processing region 266 on the top PCB layer 252, and between thecircuit regions 276 and 278 on the bottom PCB layer 254. A number ofvias carry the input power from the top PCB layer 252 throughout thethickness of the PCB assembly 250 to the bottom PCB layer 254. Thegeographic partitioning of the power input extends throughout the PCBassembly 250, as the ground partitions 268 and 280 also reach the bottomPCB layer 254 as shown in FIG. 10B.

The top PCB layer 252 also includes a control interface region 282separated from the mixed signal processing region 266 by a groundpartition 284. The control interface region 282 may include an FPGA 286and other digital components for data transfer and other processing ofthe digitized representations of the PET detector signals. The controlinterface FPGA 286 may be geographically separated from the othercomponents of the control interface region 282 by a dedicated groundpartition.

The above-described partitioning of the PCB assembly 250 may extend tothe power and/or ground planes of the PCB assembly 250. Each powerand/or ground plane layer of the PCB assembly 250 may include multiple,discrete plane structures for the above-described regions of the top andbottom PCB layers 252, 254. One or more ground planes configuredrelative to signal layers of the PCB assembly 250 may connect to one ormore power ground planes configured relative to power distributionlayers of the PCB assembly 250. Alternatively, only some of the groundplane layers of the PCB assembly 250 include the multiple, discreteground plane structures. Each ground plane structure may be dedicated toa specific region or region type. In one example, each ground planelayer of the PCB assembly 250 includes respective regional ground planestructures for the analog circuitry, for the digital circuitry, and forthe chassis ground. One of the ground plane structures may then bededicated to supporting the digital devices of the control interfaceregion 282. The other ground plane structure may then be dedicated tosupporting the analog circuitry in the detector filter interface region258, the input buffer amplifier region 260, and the analog region 278.The digital ground plane structure may be configured to support themixed signal processing region 266. Each of the above-described groundpartitions may be coupled to the chassis ground plane structure. Inanother example, an additional discrete ground plane structure may bededicated to supporting the mixed signal processing region 276. Examplesof such partitioned ground layers of the PCB assembly 250 areschematically shown in FIG. 13.

FIG. 11 shows one example of a ground partition 300 in greater detail.The ground partition 300 is defined by a plurality of vias 302 thatextend through the PCB assembly 250. The vias 302 may extend through onelayer, each layer, or a subset of layers, of the PCB assembly 250. Atthe top PCB layer 252 (FIG. 10A) and the bottom PCB layer 254 (FIG.10B), the vias 302 may be tied to chassis ground as described herein.The plurality of vias 302 of each ground partition 300 may thus form awall of chassis ground extending vertically through the PCB assembly250. Each via 302 may be filled with copper clad or one or more otherconductive materials. The vias 302, when taken collectively along one ofthe ground partitions 300, may minimize the current flowing into or outof a specific circuit or circuit region, such as one of the mixed signalprocessing quadrants or the control interface.

The plurality of vias 302 may be disposed in a grid pattern to definethe length and width of the ground partition 300. The arrangement of thevias 302 may vary from the grid pattern shown, as the vias 302 need notbe arranged in columns and rows. The vias 302 may be scattered in avariety of different configurations or arrangements, and need not form arepetitive pattern such as an array. The via arrangement effectivelydefines longitudinal boundaries 304 of the ground partition 300. Thespacing of the longitudinal boundaries 304 defines the width of theground partition 300.

The size and spacing of the vias 302 may vary across the array. Forinstance, external vias 306 along the longitudinal boundaries 304 may belarger than internal vias 308 disposed within the perimeter formed bythe external vias 306. In one example, each external via 306 has adiameter of 22 mils, while the internal vias have a smaller diameter,e.g., 20 mils. The grid pattern may position the vias 302 with acenter-to-center distance of 35 mils in the lateral direction orthogonalto the longitudinal boundaries 304, and 50 mils in the directionparallel to the longitudinal boundaries 304. The external vias 306 mayhave the same 50 mil pitch in the longitudinal direction, but a varyinglateral spacing.

The ground partition 300 may be fabricated by removing solder mask inthe area on which the ground partition 300 is formed. The solder maskmay be removed via an exposure of 100 mils and a finish of emersionsilver or gold flash. Ground planes within the PCB assembly 250 may befilled with conductive material (e.g., copper clad) along the groundpartition boundaries 304.

PET detector signals to be processed by the circuitry of the PCBassembly 250 may be routed through the ground partition 300 throughrespective lateral waveguide apertures 310. As differential signals, thePET detector signals may be routed side-by-side as a differential pair.All of the circuit signals in the PCB assembly 250, both analog anddigital, may be differential in structure. One or more signals maysingle ended or non-differential, such as the single-ended communicationsignals over the above-described i2C bus. Given the number ofdifferential signals received from the PET detectors, a considerablenumber of waveguide apertures 310 may pass laterally through the groundpartition 300. All of the differential signals need not pass through theground partition 300 on the same level. The waveguide apertures 310 maybe distributed over multiple PCB layers in the PCB assembly 250. Thenumber of waveguide apertures 310 passing laterally through the groundpartition 300 may vary given a certain ground partition, the number ofPCB layers devoted to signal planes, etc. The ground planes within thePCB assembly 250 along the ground partition 300 may be filled up to thesignal spacing limits for routing a trace through the waveguideapertures 310 laterally through the ground partition 300.

The waveguide apertures 310 may be used to allow traces to be formedwithin the waveguide aperture 310 for carrying PET detector signals fromthe input filters in the region 258 to the analog input bufferamplifiers in the region 260, crossing the ground partition 256 as shownin FIG. 10A. The waveguide apertures 310 may alternatively pass throughthe ground partition 268 to allow traces to carry PET detector signalsfrom the analog input buffer amplifiers in the region 260 to the A/Dflash amplifiers in the region 266. The waveguide apertures 310 (and thewaveguide aperture structure) may be used to pass PET detector signalsthrough any of the ground partitions of the PCB assembly 250.

The waveguide apertures 310 and the ground partition 300 are dimensionedso that EMI in the MR frequency band cannot propagate laterally throughthe ground partition. For example, the width of the ground partition andthe spacing between the vias 302 are selected so that the waveguideapertures 310 prevent signals in the MR frequency band from passing fromone circuit or circuit region to a neighboring or adjacent circuit orcircuit region.

The spacing between the vias 302 may be adjusted to accommodate some orall of the waveguide apertures 310. For example, one or more of the vias302 (e.g., rows of vias) may be removed in order to provide sufficientspacing for high voltage signals (e.g., power traces) and to addresscreepage and clearance concerns.

The ground partition 300 (and one or more of the other ground partitionsdescribed herein) may be in electrical communication with chassisground. The chassis of a DAU may include the housing in which the PCBassembly 250 is disposed, along with any supporting structure, such as amounting frame. Each DAU housing may define discrete compartments for anumber of the circuits and circuit regions described above. The housingmay thus include a number of walls, seams, or other dividers thatseparate a circuit or circuit region from one or more adjacent circuitor circuit regions. These dividers may form internal or external wallsof the DAU housing. These dividers may thus be in addition to thosewalls that define the outer border of the DAU. One or more of thesewalls, seams, or other dividers may be disposed along a respective oneof the above-described ground partitions. Each of these walls, seams, orother dividers may be in contact with the ground partition. In oneexample, each of these walls, seams, or other dividers are coupled tothe ground partition via an EMI gasket.

FIG. 12 depicts an EMI gasketing strategy that may be used to provideEMI shielding in conjunction with a DAU housing 320. The EMI strategyincorporates multiple redundancies to minimize EMI emissions andsusceptibility. A first redundancy involves overlapping walls at eachborder or seam. A second redundancy involves multiple EMI gaskets foreach seam.

The DAU housing 320 may include a chassis or base 322 and a cover 324.The base 322 and the cover 324 are configured to enclose the PCBassembly 250. The base 322 and the cover 324 may include or support oneor more structural components 325, including, for instance, a platform,brace, wall or other structure to support the PCB assembly 250. Thestructures 325 may act as a mounting frame or other support structurefor the DAU. One or more of the structures 325 may be configured aswalls that couple the ground partitions of the PCB assembly 250 tochassis ground. The base 322 and/or the cover 324 may have a number ofinternal or external walls that define a number of compartments, asdescribed above. An external wall 326 of the cover 324 is shown in FIG.12 as one example. The external wall 326 is stepped to accommodate anupstanding wall 328 that projects upward from the base 322. In thisexample, the external wall 326 includes a lower section 330 and an uppersection 332. The lower section 330 may include a flange 334 to receiveone or more bolts or other fasteners 336 for securing the cover 324 tothe base 322. The lower section 330 need not be configured as shown, andmay thus have other projections or structures configured to couple thebase 322 and the cover 324. For instance, the lower section 330 of thewall 326 may include a recessed seat to receive a bolt or otherfastener.

The wall 326 is configured to overlap the base 322 in a manner thatdefines multiple EMI gasket interfaces. In this example, a shoulder 338of the wall 326 forms a first interface with the base 322 at a top ofthe wall 328, while the flange 334 forms a second interface with anupper rim 340 of the base 322 beyond the wall 328. The shoulder 338 mayform an upper rim that extends laterally outward from the wall 328,running along the perimeter of the housing 320 adjacent to the wall 328.The walls 326 and 328 thus overlap between the two interfaces along theexterior of the housing 320. The wall overlap in this example is formedalong the height of the wall 328. Other shoulder arrangements, such as arim extending inward, may be used.

The PCB assembly 250 may be in thermal communication with the base 322and/or the cover 324 via one or more thermal gaskets 341. In the exampleshown, one or more of the thermal gaskets 341 are disposed between eachside of the PCB assembly 250 and the base 322 and the cover 324. Thethermal gasket(s) 341 may alternatively be disposed between the base 322and the PCB assembly 250.

Each interface may include respective EMI gasketing. The first interfaceat the top of the wall 328 includes an EMI gasket 342. The secondinterface beyond the wall 328 includes an EMI gasket 344. Theconstruction, materials, and other characteristics of the EMI gaskets342, 344 may vary. In one example, the EMI gaskets 342, 344 includedispensed form-in-place copper filler material commercially availablefrom made by Laird Technologies under model number SNK55-RXP orSNK60-HXP.

The EMI gaskets 342, 344 may run along the exterior of the housing 320and/or between various housing components (e.g., the housing base 322and/or the cover 324) and one or more of the ground partitions on thelayers 252, 254 of the PCB assembly 250. The EMI gaskets 342, 344 maythus enclose the PCB assembly 250. In one example, the top and bottomlayers 252, 254 of the PCB assembly 250 may include a border areaadjacent to the EMI gasket 342. The border area may be formed as aground partition such that the above-described ground partitions aretied to chassis ground as the housing 320 clamps down on the groundpartitions.

The redundant shielding provided by the EMI gasket 342 may be useful toprovide continuous gasketing along the perimeter of the PCB assembly250. The protection from the EMI gasket 342 is not interrupted ordisturbed by the assembly of the housing 320 due to the absence of anyfasteners along the interior gasket 322. In contrast, the EMI gasket 344is not continuous, as the gasket 344 is pierced by the fasteners 336.

The EMI gasketing strategy may be repeated in connection with a housingfor the power filter board (FIG. 5) to the main DAU housing. Forexample, the power filter housing may be mounted to the cover 324 of thehousing 320 such that the cover 324 acts as a base for the power filterhousing. The cover 324 may then include a wall similar to the wall 328to form an overlap with two EMI gasket interfaces, as described above.

One or more EMI gaskets may be provided for any one or more of theinternal cavities or compartments of the housing 320. In someembodiments, the DAU includes an EMI gasket for a respective one of theground partitions that links the ground partition to the housing 320.The housing 320 may not include a wall or other divider disposed uponthe ground partition to establish the chassis ground connection, as inthe case of the border area of the PCB assembly 250 near an externalwall of the housing 320. The EMI gasket may instead link the groundpartition to a component of the housing 320. The housing 320 and EMIgasket nonetheless clamp down on the ground partition as the DAU isassembled. For example, the EMI gasket may couple the ground partitionto the cover 324. One or more of the ground partitions may thus be incommunication with chassis ground. Alternatively, the housing 320 mayinclude a seam, wall or divider along one or more of the groundpartitions described above. The divider may be positioned adjacent tothe ground partition, with an EMI gasket in between the divider and theground partition. The divider and EMI gasket then clamp down on theground partition during assembly. For the ground partitions, the dualstrategy is not provided, but may be used in alternative embodiments.

Some of the ground partitions defined on the top PCB layer may not havea housing divider or EMI gasket interface disposed thereon, despitebeing configured to support such structures. For example, each FPGA maybe partitioned from the other devices in the mixed signal processingregion via the ground partitions, but not be enclosed in a respectiveon-board shield provided by, for instance, a discrete housingcompartment defined by overlapping walls. The ground partitions maystill provide separation through the above-described waveguide aperturesto reduce EMI emissions through the PCB assembly 250.

FIG. 13 shows the PCB assembly 250 in greater detail to depictindividual PCB layers of the stack according to one embodiment. The PCBassembly 250 may be assembled from multiple sub-assemblies, or subsetsof partitioned PCB layers. For example, the PCB assembly 250 may bepartitioned into three vertical sections or cores, including a topsection (e.g., layers 1-8), a middle section (layers 9-12), and a bottomsection (e.g., layers 13-16). The top section may provide interleavedsignal and ground planes for the differential signals. The signals maybe routed side-by-side as pairs as described above. The middle sectionmay have a number of partitioned dielectric layers for by-passing thetop-side circuitry. For example, the middle section may be configured toprovide by-pass capacitances formed via two regionally partitionedceramic dielectric layers. The bottom section may distribute power tolocal linear regulators and delivers power to the top-side circuits. Thebottom layer may also be configured as a thermal gasket interface to theexternal EMI housing, providing heat sinking to the bottom sideregulators as well as heat sinking to top-side circuits through theground partition vias.

In this example, layer 1 corresponds with the top PCB layer 252, whilelayer 16 corresponds with the bottom PCB layer 254. Layers 1, 3, 5, and7 may be configured as ground planes for embedded signal layers 2, 4, 6,and 8. Each ground plane may be formed with 1 oz copper, while eachsignal layer may have ½ ounce copper. Layers 1-8 are schematically shownwith a signal via 350 representative of how the differential signalpairs routed in these layers do not extend the entire depth of thestack, e.g., from the top layer 252 to the bottom layer 254. Layers 2,4, 6, and 8 carry the interconnects between the above-described deviceson the top PCB layer 252 in this embodiment. The RF signals are thusconfined to the layers 1-8. Layers 1-8 are also schematically shown witha power via 352 representative of how, in contrast to the signal pairs,power travels the depth of the stack, e.g., from the top layer 252 tothe bottom layer 254. Different numbers of ground plane and signallayers may be used. Different via depths may be used based on the signalrouting.

Any insulating substrate 354 such as FR-4 may be used to support each ofthe signal layers and ground layers of the top section of the PCBassembly 250. Each signal layer of the PCB stack is compatible with avariety of dielectric substrate materials. The signal layers are notlimited to copper foils.

Beneath the top signal routing section are several layers that formembedded dielectric structures. In this example, layers 9, 10, 11, and12 may be formed with 1 ounce copper configured as either a power groundplane (layers 9 and 12) or a power plane (layers 10 and 11). Apartitioned dielectric layer 356 may be disposed between the planes 9and 10 and has a thickness to support the formation of embeddedcapacitors. The partitioned dielectric layer 356 may be a ceramicmaterial. One example of a ceramic dielectric material suitable for thepartitioned dielectric layer 356 is “C PLY,” which is commerciallyavailable from 3M Corporation. Other dielectric materials may be used,including, for example, a thin layer of FR-4 materials (e.g., 50 um).Another C PLY (or other dielectric) layer 358 may be disposed betweenthe planes 11 and 12. Each dielectric layer 356, 358 may be used to formby-pass capacitors for the devices on the top and bottom PCB layers 252,254. Use of the C PLY-based dielectric layers provides very lowinductance capacitance for the by-pass capacitors. The capacitors may beformed between opposed conductor structures formed in the layersadjacent to a respective one of the dielectric layers 356, 358. Thebottom PCB layer 254 may include a number of discrete capacitors thatwork in conjunction with the dielectric layer-based capacitances toprovide the by-pass capacitances. Other planes 10 and 11 within themiddle dielectric section may be supported by FR-4 or other dielectricsubstrates.

The bottom section of the PCB assembly 250 may be directed to powerdistribution and regulation, and disposed on the other side of thedielectric section from the top section. The section may include layers13-16. In this example, no RF signals are routed through these layers asa result of the vertical partitioning of the PCB assembly 250 in whichsignal vias do not extend from top to bottom in the PCB stack. Each oflayers 13-15 may be formed with 1 ounce copper. Layer 16 may be formedwith 2 ounce copper. The thicker copper of the layer 16 may be directedto handling the entire current presented by one or more of the powersupplies, which may enter the PCB stack 250 at the layer 16. In oneexample, four different power buses are brought into the PCB assembly250, including two digital rails (e.g., digital high and low) and twoanalog power supply rails (e.g., +V_(A) and −V_(A)).

The bottom section may be configured to act as a thermal heat sink andthermal gasket interface for conductive heat transfer off of the PCBassembly 250. The bottom section may be in contact with a base, mount,or other structural component of the DAU housing, which, in turn, is inthermal communication with a coolant path as described above. The vias352 and bottom-section vias 360 may be configured to manage the thermalload presented by the PCB assembly 250. One or more of the vias 352, 360may be configured as PAD-in-vias.

The above-described DAU board-level partitioning provides integrated andgeographically separated shielded structures and thermal management fora number of circuits, including those directed to power distribution andregulation, embedded ceramic dielectrics, mixed-signal circuits (analogto digital converters), and analog circuits. The board-levelpartitioning limits the EMI emissions from DAU such that operation of 28housed DAUs, each consuming 100 W of power, have no emissions seeable bythe MR system having 32-bit receivers with 140-160 dB of dynamic range.

The disclosed systems may address issues such as thermal management, RFemissions and susceptibility to MR gradient and RF fields for the dataprocessing units (e.g., DAUs) located in the RF cabin. Theabove-described data processing (or acquisition) units (DAUs) are notlimited to use within an RF cabin. The DAUs are well suited forapplications or installations in which one or more of the DAUs arelocated outside of the RF cabin.

MR/PET performance may be improved by locating the DAUs proximate thePET block detectors, not only in the RF cabin, but also along anexterior side or other face of the MR scanner. The lateral side and endface locations of the DAU arrays reduce the PET signal cable lengthrelative to, for example, MR/PET systems in which the digitizationoccurs outside of the RF cabin. The lateral side and end faces locationsmay also assist in establishing radial, axial, and other symmetry of theDAU array and/or cabling associated therewith to minimize the impact onMR image quality. PET signal integrity may be improved and potentialinteroperability issues between the MR and PET DAU may be overcome.Placement of the DAU array in a lateral side region may also minimizethe length of the MR scanner (e.g., the MR bore length), while placementof the DAU in an end face location may maintain the form factor of thescanner, each of which may be useful in those installation sites havinglength or other dimensional restrictions.

The modularity of the disclosed systems may improve manufacturabilityand serviceability. The disclosed systems may be partitioned or fullyassembled and tested as a subsystem prior to integration into the MRcabin. Any combination of one or more of the aspects or embodimentsdescribed above may be used.

While the invention has been described above by reference to variousembodiments, it should be understood that many changes and modificationscan be made without departing from the scope of the invention. Forexample, the DAUs may be positioned outside the RF cabin. It istherefore intended that the foregoing detailed description be regardedas illustrative rather than limiting, and that it be understood that itis the following claims, including all equivalents, that are intended todefine the spirit and scope of this invention.

The invention claimed is:
 1. A data processing unit for an integratedmagnetic resonance (MR) and positron emission tomography (PET) system,the data processing unit comprising: an RF shield housing; a first inputport in the RF shield housing configured to receive a PET detectorsignal; a first filter disposed in the RF shield housing, incommunication with the first input port, and configured to remove MRnoise from the PET detector signal; a second input port in the RF shieldhousing configured to receive DC power; a second filter disposed in theRF shield housing, in communication with the second input port, andconfigured to remove the MR noise from the DC power; and a signalprocessing circuit disposed in the RF shield housing and powered by theDC power, the signal processing circuit including an analog-to-digitalconverter to digitize the PET detector signal.
 2. The data processingunit of claim 1, further comprising a cooling interface in thermalcommunication with the RF shield housing.
 3. The data processing unit ofclaim 1, further comprising a thermally conductive path between the RFshield housing and the signal processing circuit to remove heatgenerated by the signal processing circuit.
 4. The data processing unitof claim 1, further comprising respective input/output (I/O) ports inthe RF shield housing for a fiber optic communication link and a clocksignal.
 5. The data processing unit of claim 4, wherein the clock signalis a sine-wave clock signal.
 6. The data processing unit of claim 1,wherein the RF shield housing includes a plurality of RF shieldedcompartments.
 7. The data processing unit of claim 1, wherein the RFshield housing has opposite sides that taper.
 8. An integrated magneticresonance (MR) and positron emission tomography (PET) system comprising:an MR scanner comprising a magnet that defines an opening in which asubject is positioned, the MR scanner shielded from externalelectromagnetic interference; a set of PET detectors disposed within theopening, each PET detector being configured to generate a PET detectorsignal; and a plurality of data processing units, each data processingunit being configured for communication with a respective one or morePET detectors of the set of PET detectors; wherein each data processingunit includes digitization circuitry configured to quantify the PETdetector signals from the one or more PET detectors in communicationwith the data processing unit; wherein each data processing unit furtherincludes a DC power input port; and wherein each data processing unitfurther includes a filter in communication with the DC power input portand configured to block a frequency band used by the MR scanner duringthe scan operation.
 9. The integrated MR and PET system of claim 8,wherein each data processing unit further includes a cooling interfacein thermal communication with a discrete RF shield housing of the dataprocessing unit.
 10. The integrated MR and PET system of claim 8,wherein each data processing unit further includes a thermallyconductive path between a discrete RF shield housing and thedigitization circuitry.
 11. The integrated MR and PET system of claim 8,wherein each data processing unit further includes respectiveinput/output (I/O) ports for a fiber optic communication link and aclock signal.
 12. The integrated MR and PET system of claim 11, whereinthe clock signal is a sine-wave clock signal.
 13. The integrated MR andPET system of claim 8, wherein each data processing unit comprises adiscrete RF shield housing having a plurality of RF shieldedcompartments.
 14. The integrated MR and PET system of claim 8, whereineach data processing unit comprises a discrete RF shield housing withtapered, opposite sides such that the plurality of data processing unitsare operable to be positioned in a curved pattern around at least partof the opening.
 15. The integrated MR and PET system of claim 8, whereineach data processing unit is disposed adjacent the MR scanner.
 16. Theintegrated MR and PET system of claim 8, wherein each data processingunit is disposed along an end face of the MR scanner.
 17. The integratedMR and PET system of claim 8, wherein each data processing unit isarranged in a planar orientation along an end face of the MR scanner andtransverse to an axis of the magnet.
 18. The integrated MR and PETsystem of claim 8, wherein the plurality of data processing units aredisposed in a symmetrical arrangement relative to the opening.
 19. Amethod of positron emission tomography (PET) imaging comprising:receiving PET detector signals via a first input port in an RF shieldhousing; receiving a DC power supply through a second input port in theRF shield housing; filtering the DC power supply to remove noise; anddigitizing the PET detector signals using analog-to-digital convertersdisposed in the RF shield housing using the filtered DC power supply.